An artery is one of the tube-shaped blood vessels that carry blood away from a heart to the body's tissues and organs. An artery is made up of an outer fibrous layer, a smooth muscle layer, a connecting tissue layer, and the inner lining cells. If arterial walls become hardened due to the accumulation of fatty substances, then blood flow can be diminished. Hardening of the arteries, or loss of vessel elasticity, is termed arteriosclerosis while fatty deposit build-up is termed atherosclerosis. Atherosclerosis and its complications are a major cause of death in the United States. Heart and brain diseases are often the direct result of this accumulation of fatty substances that impair the arteries' ability to nourish vital body organs.
Balloon angioplasty is a nonsurgical method of clearing coronary and other arteries, blocked by atherosclerotic plaque, fibrous and fatty deposits non-uniformly on the walls of arteries. A catheter with a balloon-like tip is threaded up from the arm or groin through the artery until it reaches the blocked area. The balloon is then inflated, flattening the plaque and increasing the diameter of the blood vessel opening. The arterial passage is thus widened. As a result of enlarging the hardened plaque, cracks may unfortunately occur within the plaque to expose the underlying fresh tissue or denuded cells to the blood stream.
There are limitations, however, to this technique's application, depending on the extent of the disease, the blood flow through the artery, the part of the anatomy, symmetrical orientation of the plaques, and the particular vessels involved. Plaque build-up and/or severe re-stenosis recurrence within 6 months is up to 30-40 percent of those treated. Balloon angioplasty can only be characterized as a moderate-success procedure. Recently, a newer technique of inserting a metallic stenting element is used to permanently maintain the walls of the vessel treated at its extended opening state. Vascular stents are tiny mesh tubes made of stainless steel or other metals and are used by heart surgeons to prop open the weak inner walls of diseased arteries. They are often used in conjunction with balloon angioplasty to prevent restenosis after the clogged arteries are treated. Stenting technique reduces the probability of restenosis; however, the success rate is still sub-optimal. The underlying fresh tissue or damaged cells still pose as a precursor for vessel reclosures or restenosis, regardless of stenting or not.
When a clogged artery is widened, the plaque is broken up and the underlying collagen or damaged endothelium is exposed to the blood flow. Collagen has a prothrombotic property, which is a part of the body healing process. Unless the collagen or the damaged endothelium is passivated or modulated, the chance for blood vessel clotting as well as restenosis still exists. Moderate heat is known to tighten and shrink the collagen tissue as illustrated in U.S. Pat. No. 5,456,662. Therefore, it becomes imperative to post-treat vessels walls after the walls are treated with angioplasty and/or stenting procedures. Due to asymmetrical plaque deposit circumferentially, the treatment parameters at different location shall be different.
One method of reducing the size of cellular tissues in situ has been used in the treatment of many diseases, or as an adjunct to surgical removal procedures. This method applies appropriate heat to the tissues, and causes them to shrink and tighten. It can be performed on a minimal invasive fashion, which is often less traumatic than surgical procedures and may be the only alternative method, wherein other procedures are unsafe or ineffective. Ablative treatment s have an advantage because of the use of a therapeutic energy that is rapidly dissipated and reduced to a non-destructive level by conduction and convection, to other natural processes.
RF therapeutic protocol has been proven to be highly effective when used by electrophysiologists for the treatment of tachycardia; by neurosurgeons for the treatment of Parkinson's disease; and by neurosurgeons and anesthetists for other RF procedures such as Gasserian ganglionectomy for trigeminal neuralgia and percutaneous cervical cordotomy for intractable pains. Radiofrequency treatment, which exposes a patient to minimal side effects and risks, is generally performed after first locating the tissue sites for treatment. Radiofrequency energy, when coupled with a temperature control mechanism, can be supplied precisely to the apparatus-to-tissue contact site to obtain the desired temperature for treating a tissue.
To effect the optimal ablation, it requires selection of the most appropriate apparatus-to-tissue contact site as well as the most effective contact surface area with appropriate energy delivery. The conventional RF ablation catheter applies one uniform RF energy to the whole electrode means. The RF energy density (or the current density expressed as amperes per unit area) is a function of the conductivity or resistivity of the current conductor circuit. More information is shown in an article entitled "Chapter 2: Physical Aspects of Radiofrequency Energy Applications" from Huang SKS, (ed.) Radiofrequency Ablation of Cardiac Arrhythmias: Basic Concepts and Clinical Applications. Armonk, N.Y.: Futura Publishing Company Inc. 1994. In several special indications, a certain portion of the tissue requires more RF energy delivery than other portions. This is particularly true when a large electrode is used for ablating a tumor, the tumor center requires more energy input while the peripheral regions demand less energy input. A conventional ablation catheter does not provide such capability for alternating RF energy delivery to different zones of a tissue for optimal treatment without undue damage to the surrounding tissues.
Stern et al. in U.S. Pat. No. 5,743,903 discloses a catheter including a body carrying an electrode element for emitting energy to ablate tissue, the electrode element having a predetermined operating characteristic. Said catheter includes an element retaining a prescribed identification code that uniquely identifies the predetermined operating characteristic of the electrode element. Stern et al. also discloses different types of electrodes with various add-on values of resistance. However, Stern et al. does no teach the RF current delivery to electrode means that comprises a plurality of surface sections, each surface section having different surface current conductance/resistance for transmitting different electrical current density through said surface section for optimal tissue treatment.
Therefore, there is an urgent clinical need for an improved medical apparatus having the capability to effectively deliver appropriately various amount of energy to various surface sections of a target tissue for optimal tissue treatment.